The present invention relates to power plant steam chests and in particular, to a steam control valve for a power plant steam chest with an improved muffler portion. In power plants such as nuclear plants, steam control valves are utilized in the steam chest which is in fluid communication with the saturated steam coming from the steam generator. The control valve includes a plug which controls the flow of steam to the outlets of the steam chest. The plug is raised to permit flow of steam to the outlets of the steam chest. The typical steam control valve has an outer housing including a bottom muffler portion having window openings therethrough. The bottom muffler portion both guides the valve plug and attentuates valve flow noise. The bottom muffler portion of the outer housing frequently experiences mechanical damage during operation. The types of damage encountered include cracking of the muffler strut portion and wear and cracking in the the bottom ring area. It has been thought that flow-induced vibration of the valve plug and/or muffler portions is the cause of the damage. It has been found in model tests that large-amplitude plug vibrations initiate above a threshold pressure ratio (or flow rate) for a given plug lift (valve opening). Below the threshold pressure ratio the vibrations appear to have a random character such as would be expected to be caused by turbulence; while above the threshold pressure ratio they have essentially a sinusoidal character, such as would be expected from a self-excited, flow-induced vibration mechanism. Above the threshold pressure ratio, the vibration of the plug increases rapidly and heavy impacting occurs between the plug and the muffler portion. One present muffler portion bottom ring design includes a series of flow holes with an upper band without flow holes adjacent the window openings. It has been found in model tests that this design has an inherent instability threshold pressure ratio (or flow rate) for a given valve lift, above which self-excited vibration initiates. The vibration is due to modulation of the flow through the flow holes and through the annular gap between the plug and the muffler portion. The modulation is caused by the relative vibratory motions of the plug and muffler portion. These vibrations may lead to valve damage and the field damage can be reasonably explained by the self-excited vibration mechanisms identified. It has been found that the self-excited vibration mechanism acting at low valve lifts is related to pneumatic hammer, a known cuase of severe vibration of shafts supported by hydrostatic bearings that use a gaseous fluid or steam. The bottom ring portion of the prior art muffler in the area of the flow holes is similar to a hydrostatic bearing while the plug is similar to a shaft. The self-excited vibration mechanism acting at higher lifts when the plug is in the upper band of the bottom ring is due to gap modulations.